Alkaptonuria

Definition

Alkaptonuria is a rare condition in which a person's urine turns a dark brownish-black color when exposed to air. Alkaptonuria is part of a group of conditions known as an inborn error of metabolism.

Causes

A defect in the HGD gene causes alkaptonuria.

The gene defect makes the body unable to properly break down certain amino acids (tyrosine and phenylalanine). As a result, a substance called homogentisic acid builds up in the skin and other body tissues. The acid leaves the body through the urine. The urine turns brownish-black when it mixes with air.

Alkaptonuria is inherited, which means it is passed down through families. If both parents carry a nonworking copy of the gene related to this condition, each of their children has a 25% (1 in 4) chance of developing the disease.

Symptoms

Urine in an infant's diaper may darken and can turn almost black after several hours. However, many people with this condition may not know they have it until mid-adulthood (around age 40), when joint and other problems occur.

Symptoms may include:

Arthritis (especially of the spine) that gets worse over time

Darkening of the ear

Dark spots on the white of the eye and cornea

Exams and Tests

A urine test is done to test for alkaptonuria. If ferric chloride is added to the urine, it will turn the urine black in people with this condition.

Treatment

Some people benefit from high-dose vitamin C. This has been shown to decrease the buildup of brown pigment in the cartilage and may slow the development of arthritis.

Outlook (Prognosis)

The outcome is expected to be good.

Possible Complications

People with this condition also can get arthritis in adulthood. The build-up of homogentisic acid in the cartilage causes arthritis in about half of older adults with alkaptonuria.

Homogentisic acid also can build up on the heart valves, especially the mitral valve. This can sometimes lead to the need for valve replacement.

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